In order to control an internal combustion engine, an air-fuel-ratio sensor (A/F sensor) which acquires the air-fuel ratio (A/F) of the fuel-air mixture in an combustion chamber based on the concentration of oxygen (O2) contained in an exhaust gas has been used. One type of such an air-fuel-ratio sensor may be a limited-current type gas sensor.
A limited-current type gas sensor used as an air-fuel-ratio sensor may include a pumping cell which is an electrochemical cell containing a solid-electrolyte object with oxide ion conductivity and a pair of porous electrodes adhered to the surface of the solid-electrolyte object. One of the pair of the electrodes may be exposed to an exhaust gas of an internal combustion engine as a test gas introduced through a diffusion-resistance portion, and the other may be exposed to the atmosphere. Furthermore, when detecting an air-fuel ratio, the temperature of the above-mentioned solid-electrolyte object may be reaised to a predetermined temperature that is a temperature at which the solid-electrolyte object expresses oxide ion conductivity (henceforth, may be referred to as an “activation temperature”) or higher.
In the above-mentioned state, when a voltage that is a voltage at which a decomposition of oxygen begins (decomposition starting voltage) or higher is applied between the above-mentioned one electrode as a cathode and the above-mentioned other electrode as an anode, oxygen contained in a test gas may be reductively decomposed at the cathode into an oxide ion (O2−). This oxide ion may be conducted to the anode through the above-mentioned solid-electrolyte object to become oxygen, and can be discharged into the atmosphere. Such a migration of oxygen by conduction of an oxide ion through a solid-electrolyte object from a cathode side to an anode side can be referred to as an “oxygen pumping action.”
By conduction of the oxide ion in association with the above-mentioned oxygen pumping action, current can flow between the above-mentioned pair of electrodes. Current which thus flows between a pair of electrodes may be referred to as “electrode current.” This electrode current may have a tendency to become larger as the voltage applied between a pair of electrodes (henceforth, may be referred to simply as an “applied voltage”) rises. However, since the flow rate of the test gas which arrives at the above-mentioned one electrode (cathode) is restricted by the diffusion-resistance portion, the consumption speed of oxygen in association with an oxygen pumping action may come to exceed the supply rate of oxygen to the cathode soon. Namely, the reductive decomposition of oxygen in the cathode can be in a diffusion-limited state.
In the above-mentioned diffusion-limited state, even though an applied voltage may be raised, electrode current may not increase, but may become approximately constant. Such property can be referred to as “limited-current property” and the range of applied voltage at which the limited-current property is expressed (observed) can be referred to as a “limited-current region.” Furthermore, the electrode current in a limited-current region can be referred to as a “limited current”, and the extent of a limited current (limited-current value) can correspond to the supply rate of oxygen to a cathode. Since the flow rate of a test gas which reaches the cathode as mentioned above may be maintained constant by the diffusion-resistance portion, the supply rate of oxygen to the cathode can correspond to the concentration of oxygen contained in the test gas.
Therefore, when an applied voltage is set to a “predetermined voltage within a limited-current region” in a limited-current type gas sensor used as an air-fuel-ratio sensor, electrode current (limited current) can correspond to the concentration of oxygen contained in a test gas. Thus, using the limited-current property of oxygen, the air-fuel-ratio sensor can detect the concentration of oxygen contained in an exhaust gas as a test gas, and the air-fuel ratio of the fuel-air mixture in a combustion chamber can be acquired based on it.
In a limited-current type sensor as mentioned above, for example, a crack and jam of a diffusion-resistance portion, a jam of a porous electrode, and a change in the conductivity of a solid electrolyte, etc. may cause an abnormality of output characteristics (for instance, an expansion of a detection value and shrinkage of a detection value, etc.). When the abnormality of output characteristics arises in a limited-current type sensor, it may become impossible to accurately detect the concentration of oxygen contained in an exhaust gas, and it can become impossible to accurately acquire an air-fuel ratio as an air-fuel-ratio sensor.
In an air-fuel-ratio sensor using a limited-current type gas sensor, for instance, a diagnostic method in which output characteristics of an air-fuel-ratio sensor is judged as abnormal when an output that deviates from a normal range of an output from the sensor corresponding to the range of an air-fuel-ratio control in an internal combustion engine is obtained from the sensor has been known.
However, an output from an air-fuel-ratio sensor may clamp within the above-mentioned normal range. In such a case, since the output from the sensor is contained in the above-mentioned normal range even though the output characteristics of the sensor have fallen into an abnormal state, there may be a problem that the sensor was wrongly judged as normal. In the art, a diagnostic method may judge that output characteristics of an air-fuel-ratio sensor is abnormal when the output from the sensor is held within the above-mentioned normal range for a predetermined time period or more during the execution of what is called “fuel cut (FC)” which cuts off the supply of fuel to an internal combustion engine (for instance, refer to the Patent Document 1 (PTL1)). In addition, the diagnostic method may be henceforth referred to as an “FC diagnosis.”